1. Field of the Invention
The present invention relates to a computer communication system and more particularly to a modem communication system where modems communicate across standard telephone lines within a premises using frequency division multiplexing.
2. Description of the Related Art
Modem communication over telephone lines has gained widespread usage with the growing number of multi-media applications implemented using personal computers. Personal computers (PCs) now perform many different types of data transfers over telephone lines, such as accessing the "Internet," communicating data files between PCs, sending and receiving facsimiles, and acting as an answering machine by sending and receiving voice information over the telephone lines. Modems are used to convert between a computer's digital signals and analog signals that can be carried on the telephone network's analog transmission lines.
Traditionally, at the core of a modem, is a digital signal processor (DSP) integrated circuit (IC), a memory, a codec, and a data access arrangement (DAA). A codec includes a digital to analog converter (D/A) and an analog to digital converter (A/D). The modem is typically coupled to a host computer bus through standard bus interface circuitry to thereby interact with a host computer. The host computer includes a central processing unit (CPU) that generates data that is to be transmitted through the modem and to receive data that is passed through the modem.
DSP based modems generally operate by processing a relatively small number of samples from the input A/D converter and generating a similarly small number of samples to be output through the D/A converter. These systems are said to work on a "sample by sample" basis or a "symbol by symbol" basis, depending on the particular implementation. Such schemes have the benefit of minimal latency time, since the input is processed almost immediately. These schemes, however, require very high computing power which is typically provided by a dedicated DSP.
In recent years, modems have been developed that operate using DSPs implemented in software. However, although software DSPs enable sampling rates to substantially increase without a corresponding increase in hardware expenses, these software DSPs operate similar to standard hardware DSPs by sampling and transmitting signals at or slightly above the "voice band". The voice band is approximately 4 kHz and has been recognized as the voice band because telephone lines typically operate with exchange circuitry, e.g., a central office or private branch exchange (PBX), configured with low pass circuitry that cuts off signals that have been transmitted at frequencies above the voice band. Thus, modem signals are typically transmitted in the voice band and DSPs sample the signals at a minimal frequency rate, i.e., twice the rate of the highest freqency of the signal being sampled, or the "Nyquist rate".